Scheme: I would prefer the implicit scheme over the explicit onenTransient Formulation: Increase the number of iterations per time step. Also check the Courant Number(should be less than 2) and reduce the time step further if requirednDiscretization: Start with first order schemes and then proceed with second ordernMesh: Since your geometry is straightforward, I assume that the mesh is not a problem herenUnder-Relaxation Factors: You may also consider starting with slightly low values and increase it as you acheive convergencenAlso check if the boundary conditions are correctn